1. Field of the Invention
This invention concerns a process for producing synthetic silica of high purity and it particularly relates to an improvement in the process for producing highly pure silica not containing Ti.
2. Description of the Prior Art
Heretofore, silica has been utilized in a wide range of application uses as fillers in resin compositions for sealing electronic components and raw materials for multi-ingredient optical fibers, fine ceramics, optical glasses, quartz for use in electronic industry, etc.
Particularly, in the case of sealing silica used as filler in resin compositions for sealing electronic components, there is a problem of misoperation referred to as soft errors caused by elements and compounds such as uranium (U) and like other .alpha.-emitters as the density of integration is increased in LSI, SLSI, etc., accordingly, it is required that the content of the .alpha.-emitter material is kept as low as possible.
Further, there exists a demand for highly pure silica not containing impurities such as Fe, Ni, Cr, Al and Ti in addition to uranium for use as starting materials for multi-ingredient optical fibers, fine ceramics, optical glasses, quartzs for use in electronic industry, as well as to silica for sealing use.
As a process for producing synthetic silica of high purity serving to such application uses, there has been proposed a method of adding an inorganic acid or an organic acid to an aqueous alkali silicate solution or bringing an aqueous alkali silicate solution into contact with an ion exchange resin thereby preparing an acidic silica sol (Japanese Patent Laid Open No. 42217/85), purifying the acidic silica sol by using a cationic exchange resin, adding aqueous ammonia thereto to prepare a neutral or weakly alkaline silica sol, then mixing the sol and an aqueous ammonium salt solution under an alkaline condition to crystallize precipitated silica, recoverying the same by separation and synthesizing silica of high purity.
However, the synthetic silica obtained by the conventional method involves a drawback in that it still contains residual Ti at high ratio although Fe, Al, Th, U, Na, etc. are removed favorably and, accordingly, results in a problem that the residual Ti absorbs light and reduces the light transmittance, for example, where it is used in multi-ingredient optical fibers.
The present inventor has made an earnest study for such a cause and found that although those ions carrying positive charges such as Fe, Al, Th, U, Na are properly removed through ion exchange upon purifying the acidic silica sol by using a cationic exchange resin in the conventional method, Ti in the form of tetravalent metal ions like that of Si exhibits natures similar with those of Si and remains in a great amount in the acidic silica sol, and not being removed by the cationic exchange resin.